Salam,
Actually the textual context of verse 2.143 and followings begins a bit earlier, at verse 2.139.
Here is a translation based on free-minds'one, except that I modified some small part that I disagree with the original
Ummah means community in the lifetime, i.e. generation and in verses 2.141 and 2.143, generation is a better fit
Ummah wasatan in verse 2.143 does not mean a balanced community in my opinion. Wasat means at the center like an edge, so ummah wasatan means a leading edge generation, i.e. they carry a new message from God, and this explains why they can be witnessed over the people.
Kasaba means "to do", "to perpetrate", and not "to earn", see verse 4.112
I kept Qibla instead of "focal point" for the purpose of this explanation.
I kept al-Masjid al-Haram instead of "restricted Temple", as I don't fairly like this translation.
2139 Say "Do you debate with us regarding God? He is our Lord and your Lord, and we have our works and you have your works, and to Him we are believers."
2140 "Or do you say that Abraham and Ishmael and Isaac and Jacob and the Patriarchs were Jewish or Nazarenes?" Say "Are you more knowledgeable or is God?" Who is more wicked than the one who conceals a testimony with him from God? God is not unaware of what you do.
2141 That is a generation that has passed; to them is what they have done, and to you is what you have done; and you will not be asked regarding what they did.
2142 The foolish from among the people will say "What has turned them away from the Qibla that they were on?" Say "To God is the east and the west, He guides whomsoever He wishes to a straight path."
2143 And as such, We have made you a leading edge generation so that you may be witness over the people, and that the messenger may be witness over you. And We did not make the Qibla that you came on except that We may know who is following the messenger from those who will turn on their heels. It was a great thing indeed except for those whom God had guided; God was not to waste your belief. God is Merciful and Compassionate over the people.
2144 We see the shifting of your face in the heaven; We will set for you a Qibla that will be pleasing to you "You shall set your face towards the Masjid al-Haram; and wherever you may be, you shall all set your faces towards it." Those who have been given the Book know it is the truth from their Lord. And God is not unaware of what you do.
2145 And if you come to those who have been given the Book with every sign they will not follow your Qibla, nor will you follow their Qibla, nor will some of them even follow each others Qibla. And if you were to follow their desires after the knowledge that has come to you, then you would be one of the wicked.
2146 Those to whom We have given the Book know it as they know their own children; and a group of them hides the truth while they know.
2147 The truth is from your Lord, so do not be one of those who doubt.
2148 And to each is a direction that he will take, so you shall race towards good deeds. Wherever you may be, God will bring you all together. God is capable of all things.
2149 And from wherever you go out, you shall set your face towards the Masjid al-Haram; it is the truth from your Lord; and God is not unaware of what you do.
2150 And from wherever you go out, you shall set your face towards the Masjid al-Haram. And wherever you may be, you shall all set your faces towards it; that the people will have no room for debate with you, except those of them who are wicked. You shall not be concerned by them, but be concerned by Me; so that I may complete My blessings upon you and that you may be guided.
So this passage begins with the statement that the generation of Jacob has passed with all it has done, and the generation the messenger speaks to must try to move forward.
What is implied here of course is that the mores and customs that were prevalent at the time and generation of Jacob are not relevant anymore for this current generation of the 7th century.
Then the passage goes on verse 2.142 with the complaint from the foolish people What has turned them away from the Qibla that they were on?
Let us put aside this sentence for the moment, I'll come back to it, and let us examine the answer to that rhetoric question "To God is the east and the west, He guides whomsoever He wishes to a straight path."
We need to understand how the answer is related to the question, and understand the question as well.
In order to understand the answer, we need to understand what is implied by "east" (mashriq) and "west" (maghrib).
Since the passage addresses to people who want to stick to the customs of Jacob, i.e. Jewish-like people, we must understand how those could have understood the passage.
It means we have to understand the ancient Jewish thought and tradition in order to understand this correctly.
And indeed, East means the place and time where the Sun rises, while West means the place and time where the Sun sets.
In the ancient Jewish thought, the Sun was metaphorically the new messianic era, so that East meant resurrection, coming of the Messiah, and West meant Exile, destruction, idolatry.
Also in the ancient Jewish thought, when the Messiah will come, he will alleviate the Law.
So, "To God is the east and the west" means that time of destruction and time of resurrection belong to God, i.e. the decision to stick with the ancient Law or to move to a newer Law.
So somehow, the change of Qibla is related to a change of Law and customs from the time of Jacob.
In verse 10.87, we find
1087 And We inspired to Moses and his brother "You shall seek out homes for your people in Egypt; and leave your homes to be a Qibla, and uphold the Salat. And give good news to the believers."
Does it mean here that the homes of Moses and Aaron are direction of prayer ?
Nobody coming from Jewish traditions as those to whom the messenger addressed would have understood that.
Verse 2.145 makes it clear that the Qibla is something to follow, it is not itself a place.
It also makes it clear that the Qibla belongs to someone
qib'lataka the Qibla of the messenger
qib'latahum the Qibla of the opponents to messenger
Mā Bađuhum Bitābiin Qiblata Ba`đin the opponents are so divided that some don't follow the Qibla of others
following the Qibla of someone else after having received knowledge is following his desires and will make you among the wicked
The word "Qibla" comes from a verb qbl that means to be facing each other, such as to receive or accept something.
This actually fits rather well with the situation where students are facing their teacher.
So Qibla simply means teaching of the Law.
But what about verse 2.144 We will set for you a Qibla that will be pleasing to you "You shall set your face towards the Masjid al-Haram; and wherever you may be, you shall all set your faces towards it." ?
In verse 2.149 and 2.150, we read And from wherever you go out, you shall set your face towards the Masjid al-Haram.
If we take the verb walla in the proper sense, then each time you go out of somewhere you must physically turn yourself towards the Masjid al-Haram which is simply ridiculous.
So the verb walla is to be taken in a figurative meaning in this context.
Turning one's face towards al-Masjid al-Haram simply means remembering the Divine Law, and that remembrance, by itself, is already a Qibla.
"from wherever you go out" the verbe kharaja indeed means to go out, but in that context, could be well understood as "to dissent".
"from wherever you go out" or "wherever you may be" this is to be understood as whatever group you dissent from or you are in. Any case, keep remembrance of the Divine Law symbolized by the Masjid al-Haram, so that people have no argument against you.